This invention relates in general to tuning apparatus particularly adapted for use with a phase locked loop (PLL) and more particularly to coarse tuning apparatus for a phase locked loop employed in a wideband frequency synthesizer.
A frequency synthesizer is a device which produces a plurality of synthesized output frequencies where each output frequency is locked to or derived from a stable master frequency source such as a crystal oscillator, atomic clock and so on.
The frequency synthesizer has been employed in radio receivers as the local oscillator and used in many other applications. The majority of modern day frequency synthesizers utilize the phase-locked loop (PLL) as an element in the synthesizer.
The PLL includes a phase detector, a low pass filter and a voltage controlled oscillator (VCO). The phase detector essentially is a linear device and its operating characteristics along with those of the low pass filter determine the overall loop performance of the phase-locked loop. Essentially the term PLL refers to a feed back loop in which the input and feed back parameters of interest are the relative phases of the wave forms. The function of a phase detector is to track small differences in phase between the input and feedback signals and to measure the phase difference between two inputs. The output of the phase detector is then filtered by means of a low pass filter and applied to the control terminal of a voltage controlled oscillator (VCO).
The VCO provides an output frequency which is a function of the control signal applied to its control terminal. In the PLL the VCO frequency changes in a direction that reduces the phase difference between the VCO signal and the reference signal. Such a loop is said to be in phase lock or locked when the phase difference is reduced to zero.
Since the frequency synthesizer is utilized in communication and test systems, the outputs available from the frequency synthesizer must be accurately specified and controlled. Therefore, one is concerned with frequency range, frequency resolution, frequency indication, frequency error, settling time, output power, harmonic distortion, phase noise, spurious interference, wide band noise and so on. Many of these factors are also associated with the phase-locked loop. It is therefore a desire to maintain low noise in the output of such a synthesizer as well as to optimize all the above characteristics for improved operation and performance.
In order to control the frequency of a synthesizer employing a phase-locked loop, coarse tuning systems are required to pretune the VCO output to within the loop acquisition frequency range prior to locking to a new frequency. The loop acquisition frequency range is the "lock in" or "pull in" range of the loop as a frequency range that will enable the loop to lock the VCO to the reference frequency at the desired frequency. If the VCO is not tuned within the acquisition range than the PLL will not lock to the reference frequency or may lock to an incorrect frequency. Thus, it is important to coarse tune the VCO so that a reliable locking at the desired frequency is assured. This is particularly important in wideband, fast switched frequency synthesizers which may be employed for military communications or other applications as well.
The prior art is aware of such approaches and essentially has referred to these approaches as coarse steering or coarse tuning. Wideband fast switching frequency synthesizers generally require a means for rapid coarse tuning of the VCO prior to closed loop acquisition at the new frequency. In regard to this, the object is to bring the VCO as close to the desired frequency as possible and then utilize the phase-locked loop to assure that the VCO will lock to the reference at the desired output frequency. Essentially, once the transfer characteristic of the VCO is known, it is possible to use a memory that stores frequency information and with the help of a digital to analog converter and, within given resolution, steer the oscillator towards its desired final frequency.
There are problems with such a system in regard to added noise at the synthesizer output. The means for providing coarse tuning should be such that the circuit is extremely "quiet" in operation. This means that the tuning means desirably should not contribute to the output spectrum of the synthesizer by adding additional noise or additional interfering signals. Thus the coarse tuning means provided with the PLL should not add to the existing phase noise of the oscillator loop. If this happens then the output spectral purity of the synthesizer will be degraded.
Fast switching wideband synthesizers cannot make use of the popular frequency discriminator phase detectors which improve acquisition range since the discriminators are inherently slow and noisy and thus are not suitable for high speed synthesizer applications. Generally speaking a wideband synthesizer would operate at frequencies of thousands of mHz over a relatively wide range of frequencies which may be, for example, from 1000 MHz to 1800 MHz or more. For examples of typical frequency synthesizers utilizing PLL devices, reference is made to a text entitled "Frequency Synthesis By Phase Lock" by William F. Egan, 1981, John Wiley & Sons, New York.
Thus, one object of the present invention is to provide apparatus which enables one to coarse tune a PLL synthesizer which apparatus does not add noise to the synthesizer output spectrum.
A further object of the present invention is to provide a tuning apparatus for a PLL frequency synthesizer which enables high speed switching operation of the synthesizer to thereby rapidly change the output frequency over a wide frequency range.